1. Technical Field
This invention relates generally to the field of fiber optic illuminators, and more particularly to an apparatus for coupling light from the source of illumination to individual fibers, while minimizing the heat transmitted thereto.
2. History of Related Art
Fiber optic illuminators generally provide a source of intense light at the end of an optical fiber, or an optical fiber bundle. As used herein, the term "fiber" is a generic term that means any type of light guide having one or more individual channels which permit light energy to be transmitted along a distance. Thus, a "fiber" includes single and multi-stranded glass and plastic fiber optic wave guides, mirrored channels of all geometric shapes, and other identical or similar light energy transmission media. Glass fibers are heat resistant and readily available, however, they are usually quite expensive in comparison to plastic fibers, which have a soft core construction, in which the flexible core is sheathed in a thin-walled tube of heat-resistant plastic. The light-transmitting core (either solid or stranded) of such plastic fibers can be operated at temperatures as high as 140.degree. C., but after long-term use, polymer cross-linking is affected, and the core ages, discolors, and becomes brittle. In fact, this is so well known in the industry that some manufacturers recommend building "service loops" into plastic fiber installations so that the aged plastic can be cut away after time and replaced with fresh fiber that has not been placed in close proximity to the source of illumination.
Heat degradation is not the only problem encountered when using plastic fibers. There is also the difficulty of infrared and ultraviolet radiation. Commonly available halogen and metal halide lamps used to illuminate the ends of fibers produce a substantial amount of infrared and ultraviolet energy. The infrared energy is mostly dissipated after a few inches of travel down the fiber, however, the ultraviolet energy travels with the light and goes along the length of the fiber, damaging the entire length of the fiber by affecting the cross-linking ability of the polymers used to make it.
Various approaches have been attempted to provide sufficient light to the fiber ends, while preventing the fibers from overheating. Such thermal control techniques include the use of dichroic reflector lamps, defocusing the lamp image, cooling fans, infrared-reflecting dichroic mirrors, and optically-tuned heat absorbers.
In one method a fan-assisted heat dissipator, comprising a fan and motor, are mounted behind the light source and draw external air through passages within a heat sink, which in turn surrounds an optical fiber. A glass rod is interposed between the end of the fiber and the source so as to prevent direct reception of the focused beam at the fiber end surface. In operation, the fan serves to draw external air through the heat sink, so as to cool the glass rod and the source. The cooling air passes onward, and is exhausted. To reduce the amount of cooling required, the glass rod serves as a relatively non-heat conductive medium for the transmission of light. However, a significant amount of optical attenuation occurs with this particular implementation, wherein several dissimilar interfaces have been interposed between the source, and the end of the fiber. Such dissimilar material interfaces are problematic because the optical output angle of the glass rod differs from the optical acceptance angle of plastic fiber, which impedes light transmission.
When multiple fibers are illuminated, another approach has typically been employed. In this case, air is typically drawn through the interstices of several unitary fibers, or stranded fibers, of a fiber bundle heat dissipator. A heat sink body, having fins, serves to radiate some of the heat in the fibers, which are clamped together within a collar, surrounded by a strap. A tightening screw is used to help maintain alignment of the fiber bundle within the heat sink body, but over-tightening often results in deformation of the individual fibers. This particular method may be used in conjunction with a glass rod, as described above. However, using such a system results in an undesirable reduction of the light received by each of the fibers within the heat sink body.
Even when multiple fibers are securely clamped together, another problem arises. Unless the fibers are somehow individually secured, those located toward the inner portion of the bundle tend to slide out of place each time the bundle is moved. The result is that the fibers at the center of the bundle often move into a less than optimal position for reception of source illumination. Another difficulty with such a simple compression arrangement is that some configurations of fiber bundling lend themselves to irregular compressive forces and result in damage to individual fibers, or distortion and uneven transmission of light. Finally, if several fibers in the bundle are of unequal diameter, a simple compression arrangement to contain the fibers within the heat dissipation apparatus usually fails to evenly distribute pressure among the fibers and contributes to distortion and/or a reduction in the amount of light transmitted by the fibers from the source to the ultimate destination.
Therefore, what is needed is a heat dissipation apparatus, or "cold coupling" apparatus, and method which act to effectively filter out infrared and ultraviolet radiation, while employing the least number of optical interfaces along the path from the source of illumination to the end of the fibers. It is further desired to provide such an apparatus and method that do not deform bundles of fibers due to excessive clamping force, and which effectively retains fibers in place within the apparatus. Further, it is desirable to provide such an apparatus and method which maintain the temperature of plastic fibers at 140.degree. C. or less, without the use of a fan, in conjunction with commonly available illumination sources.